This invention relates to a process for preparing an aqueous aminoplastic urea-formaldehyde resin in order to be used as thermosetting adhesive for lignocellulosic materials.
Urea-formaldehyde (UF) resins are produced by a condensation polymerization reaction between urea and formaldehyde. These UF polymers have proven to be very good adhesives and due to their high reactivity and cost efficiency, are the most popular binders for a wide range of industrial sectors. Primary position among them appears to have the wood based panels manufacture where the UF resins are applied in a wide range of products such as particleboards, plywood, oriented strand boards (OSB) and various fibreboards with main representative the medium density fibreboards (MDF). Wood based panels work for a broad variety of applications in outdoor and indoor housing construction (especially OSB, plywood and particleboard), and furniture manufacturing (especially MDF and particleboard).
Urea-formaldehyde resins due to their extensive use have to perform according to a wide range of requirements. They must withstand the stress in the glued product, tolerate the climate exposure, be environmentally friendly and easy to apply, be cost efficient and fulfill the performance requirements given by different standards. These necessities coupled with the continuous demand for higher production capacity and higher quality glue bond performance at the same time, make the development of new adhesive systems very challenging.
Till in the middle of nineties the UF resins had F/U mole ratio ranging from 1.8 to 1.4 but today resins with lower molar ratios are required as to comply with the current stricter health and safety regulations. In this trend, many paths have been explored while particular emphasis was given in changes to resin formulations mostly with reference to lowering the formaldehyde-to-urea mole ratios (F/U). This is because even a small decrease like from 1.5 to 1.1 yields approximately a 10-fold decrease in free formaldehyde in the panel. Of course, many other factors are also liable for formaldehyde emissions. The released formaldehyde from panels can originate from the degradation of incompletely cured resin, or resin components, such as methylolurea, the formaldehyde which was bound to wood cellulose during the hot-press cycle and which slowly hydrolyses under the influence of the acidic humidity in the wood as well as the bulk resin degradation. The UF resins contain methylol groups, methylene ether bridges and other reaction products, which can hydrolyse back to formaldehyde. The oxygen-free methylene linkage is the most resistant to hydrolysis while the weakest links existent in the cellulose-resin link are the hemiacetals, ethers and methylols. To fabricate effective UF resins of very low F/U mole ratio is a challenge for the scientists as this reduction weakens the internal bond in board and in general deteriorates its mechanical properties.
In the conventional procedure used for the commercial manufacture of urea-formaldehyde resins, for use in adhesives, textiles, paper, coatings, agricultural and other applications, the resins are prepared by the reaction of urea and formaldehyde under neutral to weakly alkaline conditions and at high formaldehyde-to-urea (F/U) molar ratio (1.8-2.5:1.0) to form methylolureas. Poly-condensation is then followed by further heating at a reflux temperature and under acidic conditions in a range of pH from 4.0 to 5.5 until the desired degree of water insolubility or viscosity is reached. The product is neutralized and may then be dehydrated, if necessary and blended with a further quantity of urea to produce a product of the desired physical characteristics. Such conventional resins usually have a final formaldehyde to urea (FU) mole ratio of from 1.1 to 1.5:1.
Being scientifically known that mixing formaldehyde and urea under strong acidic conditions favours mostly the creation of methylene bridges than ether links, Williams, J. H., in U.S. Pat. No. 4,410,685 (1983), “Hydrolytically stable urea-formaldehyde resins and process for manufacturing them”, and U.S. Pat. No. 4,482,699 (1984) “Low emitting aqueous formulations of aminoplast resins and processes for manufacturing them”, proposes the production of thermosetting resins by starting the reaction of urea and formaldehyde at higher F/U molar ratio (2.5-4.0:1.0) but under severe acidic conditions (pH<1). During the evolvement of the synthesis procedure the mixture is heated at a reflux temperature and the pH is gradually shifted to the alkaline side together with sequential urea increments in order to decrease the F/U molar ratio to the desired value. This high temperature is maintained throughout the polymerisation stage while pH is lowered at mild acidic values till a Gardner viscosity of T/U is reached. This process is claimed to yield in a resin with F/U molar ratio of 1.0-2.3:1.0 having high resistance to hydrolysis and thus low formaldehyde emission. However, this process necessitates a careful control of the reaction conditions, which is difficult especially at the industrial scale, to prevent resin gelling with subsequent blocking of the apparatus.
Resins having an F/U molar ratio as low as 1.0:1.0 have been described in British Patent Specification No. 1420017. These resins said to be suitable for binding wood-based materials and are prepared by condensing formaldehyde and urea at a F/U molar ratio of at least 3.0:1.0 and pH not more than 3.0. When 1-15% of the present urea is combined in the form of urons a sufficient amount of urea is added to lower the F/U molar ratio to 1.0-2.75:1.0. Hereafter pH is adjusted to 5.0-6.5 or 8.0-10.0 and reactants are condensed until the desired product is formed. A serious disadvantage of this process is that on an industrial scale it is difficult to obtain a consistent product and there is a serious risk of the product forming an intractable gel during the process, requiring dismantling of the apparatus in order for it to be cleaned. Further when the product has been successfully made it has poor “wash down” properties. This means that pipes, pumps etc. through which the resin has been passed, are difficult to clean, due to the inherent stickiness of the resin.
Ian R. Whiteside, in U.S. Pat. No. 4,968,773 (1990), has proposed the preparation of a urea-formaldehyde resin having low total extractable formaldehyde. According to his description, a mixture of formaldehyde and urea having formaldehyde-to-urea (F/U) molar ratio 2.0-3.0:1.0 and pH 6.0-11.0 is heated up to 80° C. These pH and temperature conditions are maintained for 10-20 minutes to allow methylolation to proceed. The following polymerisation stage is taking place at the temperature of 80° C. and pH 0.5-3.5 while after the desired viscosity has been reached the mixture is neutralised to pH 6.5-9.0. The mixture is cooled down to 25-45° C. and a final charge of urea is added for a formaldehyde-to-urea molar ratio within the range of 0.8-1.8:1.0. The drawback with this process is that the polymerisation procedure is taking place at very high F/U mole ratio (2.0-3.0:1.0) and thus very large amounts of urea are required as post additions at the end of the procedure where the cooking parameters (low temperature and alkaline pH) do not permit a substantial linkage of urea molecules to the structure of the already formed polymer. Thereupon the utilisation of urea present in the final product is insufficient and large amount of formaldehyde remains unlinked which is in strict correlation with the formaldehyde emissions from the boards prepared with this resin.